FIGS. 1A and 1B depict conventional magnetic elements 10 and 10′. The conventional magnetic element 1 is a spin valve 10 and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional nonmagnetic spacer layer 16 and a conventional free layer 18. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. The conventional nonmagnetic spacer layer 16 is nonmagnetic and conductive. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional insulating barrier layer 16′ and a conventional free layer 18′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
Depending upon the orientations of the magnetizations of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetizations of the conventional free layer 18/18′ and conventional pinned layer 14/14′ are parallel, the resistance of the conventional magnetic element /10′10 is low. When the magnetizations of the conventional free layer 18/18′ and the conventional pinned layer 14/14′ are antiparallel, the resistance of the conventional magnetic element 10/10′ is high.
To sense the resistance of the conventional magnetic element 10/10′, current is driven through the conventional magnetic element 10/10′. Current can be driven in one of two configurations, current in plane (“CIP”) and current perpendicular to the plane (“CPP”). In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 10/10′ (up or down as seen in FIG. 1A or 1B).
One of ordinary skill in the art will readily recognize that the conventional magnetic elements 10 and 10′ may not function at higher memory cell densities. The magnetic field required to switch the magnetization of the free layer 18 or 18′ (switching field) is inversely proportional to the width of the conventional magnetic element 10 or 10′, respectively. Because the switching field is higher for smaller magnetic elements, the current required to generate the external magnetic field increases dramatically for higher magnetic memory cell densities. Consequently, cross talk, power consumption, and the probability that nearby cells will be inadvertently switched may increase. The driving circuits used to drive the current that generates the switching field could also increase in area and complexity. This upper limit on the write current amplitude can lead to reliability issues because some cells are harder to switch than others (due to fabrication and material nonuniformity) and may fail to write consistently.
In order to overcome some of the issues associated with magnetic memories having a higher density of memory cells, spin transfer may be utilized to switch the magnetizations 19/19′ of the conventional free layers 10/10′. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic element 10. Current knowledge of spin transfer is described in detail in the following publications: J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, vol. 54, p. 9353 (1996), and in F. J. Albert, J. A. Katine and R. A. Buhrman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809 (2000). Thus, the following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. As a result, a spin-polarized current can switch the magnetization 19′ direction of the conventional free layer 18′ if the current density is sufficiently high (approximately 107–108 A/cm2) and the lateral dimensions of the spin tunneling junction are small (approximately less than two hundred nanometers). In addition, for spin transfer to be able to switch the magnetization 19′ direction of the conventional free layer 18′, the conventional free layer 18′ should be sufficiently thin, for instance, preferably less than approximately ten nanometers for Co. Spin transfer based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10/10′ are small, in the range of few hundred nanometers. Consequently, spin transfer is suitable for higher density magnetic memories having smaller magnetic elements 10/10′.
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization 19′ of the conventional free layer 18′ of the conventional spin tunneling junction 10′. For example, the magnetization 19′ of the conventional free layer 18′ can be switched from a direction antiparallel to the magnetization of the conventional pinned layer 14′ to a direction parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). Thus, the majority electrons traveling from the conventional pinned layer 14′ have their spins polarized in the same direction as the magnetization of the conventional pinned layer 14′. These electrons may transfer a sufficient portion of their angular momentum to the conventional free layer 18′ to switch the magnetization 19′ of the conventional free layer 18′ to be parallel to that of the conventional pinned layer 14′. Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to the magnetization of the conventional pinned layer 14′ to antiparallel to the magnetization of the conventional pinned layer 14′. When current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction), majority electrons have their spins polarized in the direction of magnetization of the conventional free layer 18′. These majority electrons are transmitted by the conventional pinned layer 14′. The minority electrons are reflected from the conventional pinned layer 14′, return to the conventional free layer 18′ and may transfer a sufficient amount of their angular momentum to switch the magnetization 19′ of the free layer 18′ antiparallel to that of the conventional pinned layer 14′.
In addition, ballistic magnetoresistance (ballistic MR) may be used to improve the signal from a magnetic element, particularly for smaller magnetic element sizes. The ballistic MR effect arises from nonadiabatic spin scattering across very narrow (atomic scale) magnetic domain walls trapped at nano-sized conditions. Current knowledge of ballistic MR is described in detail in the following publications: G. Tatara, Y.-W Zhao, M. Munoz and N. Garcia, “Domain Wall Scattering Explains 300% Ballistic Magnetoconductance of Nanocontacts”, Physical Review Letters, vol. 83, p. 2030 (1999); Harsh Deep Chopra, and Susan Z. Hua, “Ballistic Magnetoresistance over 3000% in Ni Nanocontacts at Room Temperature”, Phys. Rev. B, vol. 66, p. 020403-1 (2002); N. Garcia, M. Munoz, V. V. Osipov, E. V. Ponizovskaya, G. G. Qian, I. G. Saveliev, and Y.-W. Zhao, “Ballistic Magnetoresistance in Different Nanocontact Configurations: a Basic for Future Magnetoresistance Sensors”, Journal of Magnetism and Magnetic Materials, vol. 240, p. 92 (2002). The description of ballistic MR included herein is based upon current knowledge and is not intended to limit the scope of the present invention.
FIG. 2A depicts a conventional conductor 30 having a length, l, that is greater than the mean free path of the charge carriers, which are typically electrons. The paths 32 of some electrons through the conventional conductor 30 are also shown. The paths 32 may be generated when current is driven through the conventional conductor 30 along its length. As the electrons traverse the conventional conductor 30 along the length, each electron typically undergoes one or more scattering events. As a result, the paths 32 of the electrons zigzag, as shown in FIG. 2A.
FIG. 2B depicts a conventional conductor 30′ having a length, l, that is less than the mean free path of the charge carriers, typically electrons. The paths 32′ of some electrons through the conventional conductor 30′ are also shown. The paths 32′ may be generated when current is driven through the conventional conductor 30′ along its length and the length of the conductor 30′ is less than the mean free path of the electrons. Because the conventional conductor 30′ is shorter than the electron mean free path, the trajectory of the electrons is ballistic in nature. Consequently, the paths 32′ are relatively straight.
Ballistic MR results in a large fractional change in resistance between two magnetic regions having different orientations of magnetization. In conventional materials, regions of different magnetization orientation are termed domains and are separated by a domain wall. In the domain wall, the magnetization of the magnetic material changes to match that of the next domain. Domain walls typically have a thickness of one hundred to two hundred nm or greater, which is greater than the mean free path of electrons in the magnetic material. Consequently, ballistic MR does not typically contribute to the resistance of a magnetic material because the reflections of electrons at the domain wall are negligible.
In contrast, FIG. 2C depicts a magnetic element 40 including two magnetic electrodes 42 and 46 that are composed of magnetic material and coupled through a nano-contact 44. The nano-contact 44 is ferromagnetic and has a length, l, that is less than the mean free path of charge carriers in the nano-contact 44. In the case where the magnetic electrodes 42 and 46 have opposite alignment, a domain wall is confined within the nano-contact 44. Thus, the magnetization changes direction in the nano-contact 44, as depicted in FIG. 2C. When the magnetizations of the magnetic electrodes 42 and 46 are aligned antiparallel, the resistance of the magnetic element is a maximum because the charge carriers passing through the nano-contact 44, which includes the domain wall, experience an extremely large scattering moment. In contrast, when the magnetizations of the magnetic electrodes 42 and 46 are aligned parallel, there is no domain wall confined within the nano-contact 44. When the magnetizations of the electrodes 42 and 46 are parallel, therefore, the resistance of the magnetic element 40 is a minimum because charge carriers have a path that is ballistic in nature. For a conventional magnetic element 40, the ballistic MR is extremely large, in some cases well over one hundred percent change in resistance between the maximum and minimum resistances.
Although ballistic MR may function, one of ordinary skill in the art will readily recognize that the magnetic elements are typically written using a magnetic field. In such a case, many of the drawbacks of conventional magnetic elements, such as the magnetic element 10 and 10′, are still present.
Accordingly, what is needed is a system and method for providing a magnetic memory element which can be used in a memory array of high density, low power consumption, low cross talk, and high reliability, while providing sufficient readout signal. The present invention addresses the need for such a magnetic memory element.